1. Field of the Invention
This invention relates generally to motor controllers and is concerned more particularly with circuitry having pulsed energizing means for controlling rotation of an anode target in an X-ray tube envelope.
2. Discussion of the Prior Art
A rotating anode type of X-ray tube generally comprises a tubular envelope having therein an electron emitting cathode disposed to beam electrons onto an aligned focal spot area of a rotatable anode target. Most of the electron energy incident on the focal spot area is converted into heat energy which could damage the focal spot area if allowed to become excessive. However, during operation of the tube, the anode target is rotated at a suitable speed for moving successive discrete areas of an annular focal track portion of the target through the focal spot area aligned with the cathode. As a result, each of the discrete areas of the focal track is rotated out of the focal spot area for a sufficient interval of time to dissipate the heat energy received in the focal spot area, whereby damage due to excessive heat energy is avoided.
Generally, the anode target is supported for axial rotation on one end portion of a heat restrictive stem which has an opposing end portion connected to a rotor of an alternating current type of induction motor disposed axially within a neck portion of the envelope. The stator of the induction motor usually is disposed externally of the envelope and in encircling relationship with the rotor within the neck portion of the envelope. During operation of the tube, an alternating current is passed through the field windings of the stator to establish a rotating magnetic field within the neck portion of the envelope to rotate the anode target relative to the cathode.
The field windings of the stator may be energized initially at an input power level sufficiently high to overcome the stationary inertia of the rotor assembly and to accelerate the assembly to a selected speed of rotation. Then, in order to avoid overheating of the enclosure in which the tube assembly is supported, the field windings may be energized at a reduced input power level sufficient to sustain the selected rotational speed of the rotor and anode target. In the prior art, this objective has been achieved by utilizing large power mechanical relays which, after a predetermined energizing time interval has elapsed, connect in the energizing circuit a stepdown transformer or large wattage resistors for reducing the input power level supplied to the field windings of the stator. However, because of size and heat dissipation requirements it has been found that the large mechanical relays and step-down transformer or large wattage resistors generally are unsuitable for lightweight portable X-ray generators.